Relational theory of learning.

Pigeons are trained to associate a red light with 1 peck at a target, and a green light with 20 pecks. The reward was the same amount of food.

After this training, pigeons the green light about 70% of the time, indicating the birds valued the same reward (food) more if more effort was required to obtain it.

J Exp Anal Behav. 2007 July; 88(1): 131149.